Optical communication technology has been improving rapidly due to the development of optical fiber technologies and light sources such as semiconductor lasers. In particular, wavelength division multiplexing, in which optical signals having different wavelengths are transmitted through a single mode fiber, has been established as a key technology in optical communication. Further, the recent development of an Erbium-doped fiber amplifier (“EDFA”) resolves the problem of energy loss in optical signals caused by long distance transmission.
In the technical field of optical communication, a wavelength band ranging from 1,530 to 1,565 nm is commonly employed. In cases where optical signals in the wavelength band are multiplexed and transmitted through a single optical fiber, each of the optical signals has a different refraction index with respect to each wavelength. The different refractive indices to the optical fiber depending on the wavelength causes dispersion, in which the optical signals through a single optical fiber over a long distance become spread along the time axis. As the required transmission distance becomes longer, the dispersion effect becomes even more prominent to the degree that the transmitted optical signals overlap each other. Thus, it is difficult to discriminate the optical signals at the receiving end of the optical transmission system.
A tunable dispersion compensator adopting an optical fiber grating has been mainly used to compensate for the dispersion of these optical signals. Such dispersion compensator facilitates a connection to an optical cable, provides low transmission loss, and offers no nonlinear phenomenon of the optical signals. For instance, if a central wavelength of the optical signals is λ1, then the optical signals consist of a plurality of wavelengths that exist within the range from λ1−δ nm to λ1+δ nm In such a case, it is known that the longest wavelength (i.e., λ1+δ nm) of the optical signals causes the most severe dispersion along the time axis. This is due to a slower transmission rate than other wavelengths when its transmission distance becomes longer. On the other hand, the smallest wavelength (i.e., λ1−δ nm) of the optical signals causes the lowest dispersion due to a more rapid transmission rate than other wavelengths even though its transmission distance becomes longer. Consequently, in order to compensate for the dispersion of said longest wavelength of the optical signal pulses, it may be desirable to reduce a reflection path in the interior of the optical fiber grating. In order to compensate the dispersion of the shortest wavelength, however, it may be preferable to extend the reflection path within the optical fiber grating. This is to compensate the dispersion of the optical signal pulses caused by the long distance transmission.
Generally, the methods of controlling the dispersion value with the tunable dispersion compensator may be classified into two methods. According to the first method, (1) the optical fiber grating is divided into several or dozens of parts, and (2) the refractive index of the grating is changed by heating and cooling each part at a different temperature in order to adjust the dispersion value. However, the variation of refractive indices of the grating parts becomes discontinuous due to the repeated heating and cooling. Further, unexpected variations of refractive indices on adjacent parts may occur due to thermal conductions. Thus, the performance of the tunable dispersion compensator becomes degraded such that it cannot be frequently used.
According to the second method, (1) optical fiber grating is attached onto a surface of a plate, (2) the plate is bent to change the period of the grating, and (3) the dispersion value is adjusted due to the changed period. A bending process is performed in the second method. More specifically, one end of the metal plate, to which the chirped optical fiber grating is attached, becomes fixed, while the other end of the metal plate is moved so that the metal plate can be bent. Therefore, the period of the chirped optical fiber grating may vary due to the tensile force and contractile force induced by bending the metal plate. In other words, the period of the optical fiber grating becomes longer when the tensile force is induced, while the period of the optical fiber grating becomes shorter when the contractile force is induced. The dispersion value, which is defined as a variation of the group delay time of wavelengths of the optical signals, can be therefore adjusted by varying the period of the optical fiber grating.
However, the second method is deficient in that a central wavelength of an optical signal, which is reflected from the chirped optical fiber, varies in accordance with the changed central period of the chirped optical fiber gratings. This is because only one end of the metal plate is moved in the conventional dispersion compensator in order to vary the period of the optical fiber grating.